Vacuum may be used in a vehicle to apply motive force in vehicle systems. For example, vacuum may be used to apply vehicle brakes, operate a turbocharger waste gate, adjust valve positions in heating and ventilation ducts. However, vacuum in vehicle systems is becoming a less available resource due to the trend of engine downsizing and variable valve timing to improve vehicle fuel economy.
One of the more significant consumers of vacuum in a vehicle is the vehicle brake system. Vacuum is used in a brake booster to apply brakes. In particular, vacuum is applied to both sides of a brake booster diaphragm when brakes are not applied. Pressure equalization across the diaphragm allows the diaphragm to return to a position where a piston in the master cylinder does not increase brake line pressure. When the brakes are applied, vacuum on a working side of the diaphragm is displaced with ambient air while vacuum remains present on the vacuum side of the diaphragm. Consequently, a pressure differential is produced across the diaphragm that motivates the diaphragm to apply force to the piston in the master cylinder, thereby increasing brake pressure and applying the brakes.
During vehicle braking, a driver receives visual and physical cues that allow the driver to know whether or not a proper amount of force is being applied to the brake pedal to provide the desired braking amount or level. However, when the vehicle is stopped, the driver receives much less information regarding whether or not braking force is adequate or more than is desired to keep the vehicle from moving. Consequently, the driver may apply more brake force than is desired to keep the vehicle from moving. As a result, more vacuum than is desired may be consumed when the vehicle is stopped.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for conserving vacuum, comprising: providing vacuum to a brake booster to apply vehicle brakes when a vehicle is stopped; and reducing or stopping brake booster vacuum consumption in response to a vacuum level in a working chamber of the brake booster being less than a threshold vacuum level that stops the vehicle from moving.
By selectively allowing air to flow into a vacuum brake booster when a vehicle is stopped or traveling at less than a threshold vehicle speed, the technical result of conserving vacuum in a vacuum system may be provided while at the same time effective braking is supplied to a vehicle. For example, if a driver is requesting more brake force than is required to hold a vehicle in place, air flow into a working side of a vacuum brake booster may be reduced or limited so that vacuum is not consumed by the vacuum brake booster. In one example, vacuum in the vacuum brake booster working chamber is allowed to be limited to a threshold vacuum that holds the vehicle in a stopped state while the brake pedal force is increased. Limiting the air entering the working chamber reduces vacuum consumption. Additionally, limiting the stroke of the brake booster diaphragm reduces vacuum consumption. By limiting air entering the working chamber, the amount of vacuum assist provided to the braking system is reduced. In some examples, vacuum assist may be limited while the driver may be allowed to apply as much unassisted master cylinder force and brake line pressure as he or she wishes. Whether or not the specific brake system design allows unassisted brake force, limiting the brake booster working chamber air pressure limits braking force and brake booster vacuum consumption. Though limited, the brake force allowed is sufficient to stop the vehicle from moving via engine torque and/or road grade. In one example, the method is carried out during stopped vehicle conditions when vehicle speed is zero and a driver is intending to maintain zero vehicle speed.
The present description may provide several advantages. In particular, the approach may conserve vacuum in a vehicle so that the vehicle's engine operates for less time at low intake manifold pressures. The approach may also conserve fuel since the engine may be able to operate more efficiently at higher intake manifold pressures for longer periods of time. Additionally, the approach conserves vacuum responsive to vehicle operating conditions such as road grade and vehicle mass.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.